Method and apparatus for inspecting a semiconductor chip prior to bonding

ABSTRACT

A chip handling apparatus, unit and method is presented. The chip handling apparatus comprises a chip supply station; a chip mounting station; and one or more chip handling units configured to pick a chip from the supply station, transport the chip to the mounting station, and place the chip at a mounting location; wherein each chip handling unit is configured to temporarily retain the chip in a defined position relative to the chip handling unit. The chip handling apparatus further comprises means for inducing sonic vibrations in the chip when retained by one of the chip handling units; and means for measuring the vibrations induced in the chip.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Swiss Patent Application No.01265/11 filed Jul. 31, 2011, the content of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention pertains to the field of automation technology. Itrelates to an apparatus, a unit, and a method for handling chips, inparticular semiconductor dies, in accordance with the preamble of theindependent patent claims.

BACKGROUND OF THE INVENTION

One of the biggest challenges in semiconductor packaging is picking,handling and processing of very thin semiconductor chips. Thesemiconductor chips are generally provided on a carrier tape, whichgenerally contains a whole semiconductor wafer that has been cut intosmall chips, a process frequently referred to as dicing. Thesemiconductor chips are thus commonly referred to as dies. The carriertape is often the same tape that supported the wafer during dicing,often referred to as dicing tape. Die thicknesses of down to 25 μm arecommon today, and there is an ongoing trend to decrease thicknesses evenfurther. Thicknesses of 15 μm have already been anticipated onsemiconductor manufacturer roadmaps.

Die bonders pick a single die from the carrier tape and place andsubsequently attach the picked die onto a substrate or onto another die.In most situations, the die is attached permanently, but configurationswhere dies are only attached temporarily also exist. Often, a temporaryattachment is subsequently turned into a permanent one by an additionalheating and/or pressing process. Typically, thin dies come with a waferbackside lamination (WBL) or film over wire (FOW) lamination, i.e. anadhesive film which is applied to an unstructured side of the wafer ordie. A process of thinning wafers, applying an adhesive film to thewafers, mounting the wafers to a carrier tape and frame, and dicing theminto individual chips is usually referred to as wafer preparation. Thelamination, which allows for the dies to be attached due to its adhesiveproperties, may be provided either between the carrier tape and thewafer, or on a surface of the wafer facing away from the carrier tape.

Picking, placing, and transport between pick and place locations may becarried out by a single chip handling unit in the die bonder. In moderndie bonders, however, a plurality of chip handling units is oftenpresent: In general, picking takes place from a so called wafer table,and is assisted by a zeroth chip handling unit, also referred to as dieejector. The die ejector facilitates the removal of the die from thecarrier tape, e.g. by pushing the die against a first chip handling unitcalled pick unit—from underneath the carrier tape. The pick unitsubsequently picks the die from the carrier tape in a pick process andhands it over to a second chip handling unit, also referred to as placeunit. The place unit places the die onto a target position, where it isattached. In some cases, at least one third chip handling unit—a socalled transfer unit—is provided to hand the die from the pick unit tothe place unit. An example is given in WO 07118511 A1 which is herebyincorporated by reference in its entirety. In this manner, a die may beattached to a substrate, e.g. a leadframe, printed circuit board,multilayer board etc., or to another die, which itself may be have beenattached in the same way.

The fabrication of very thin wafers and the corresponding dies is veryexpensive as compared to standard thickness wafers without lamination.Sawing, picking as well as handling of very thin dies have significantyield losses. Most of the yield is lost during wafer preparation, diepicking, or subsequent handling, resulting in damaged dies due totypical defects as e.g. broken dies, cracked dies, chipped dies, etc.Both place and attachment processes, in comparison, are more reliable,giving rise to only negligible loss.

In particular for stacked die bond processes where two or more dies areattached onto one another, attaching of a broken die may have dramaticconsequences: If a damaged die is attached onto a stack, all previouslyattached dies in that stack—also referred to as package—are lost. In anextreme situation, for example, a package consisting of 15 stacked diesmay thus be lost by attaching a broken 16th die on top of it. Assuming apick process yield of 99%, an expected package yield drops to 0.99^16,i.e. to 85% in this example, and to only 44% for a pick process yield of95%.

Known inspection methods for detecting broken, cracked or chipped dieson die bonders are performed before the pick process—which itself haslimited yield—and are generally based on imaging a die surface undersurface illumination, followed by image processing. These inspectiontechniques have limited reliability due to low crack contrast and crackwidth, die warpage—also referred to as potato chip effect—and due to theinterference of crack signatures with other similar looking patterns onthe die surface.

Until recently, die bonders have not offered any mechanism orfunctionality to detect die cracks prior to the bond process and afterthe pick process. There were two main reasons for that: First, thetypical “direct” pick & place architecture of die bonding machines doesnot allow for the usage of special (pick & place) tooling whichincorporates means for die crack detection. Second, die cracks are hardto detect as their appearance varies extremely, both in shape, widthetc.

In WO2011/018375 A1, a method and apparatus for inspecting a chip priorto bonding by means of an optical crack detection method was described.

SUMMARY OF THE INVENTION

It is thus an object of the invention to allow for an inspection afterpicking of a die rather than—or in addition to—one before picking of thedie, e.g. on a die ejector, and thus guarantee that only undamaged diesare attached to a substrate or a previously attached die by a diebonder.

In addition, the invention shall allow for an identification of damageddies before attach, and thus to allow for omitting to place such damageddies onto a substrate or a stack of previously attached dies. Ideally,inspection of the die should be possible immediately before it is placedand subsequently attached to the substrate or to an already attacheddie.

The above objects are achieved by a chip handling apparatus and aprocess for handling a chip according to the independent claims.

In a chip handling apparatus and a method for handling a chip inaccordance with the present invention, acoustic and/or ultrasonicvibrations are induced in a die, e.g. by means of an actuator, inparticular a piezo electric actuator. In a preferred embodiment of themethod and apparatus in accordance with the invention, the die iscentered on the actuator and temporarily attached by means of vacuum. Asan excitation frequency, i.e. a vibration frequency of the actuator, issweeped, eigenmodes of the die are excited. Eigenfrequenciescorresponding to the eigenmodes may be characterized by a mode number,and typically range from a few 100 Hz to several 10 kHz, depending ondie size, die thickness, etc.

In an exemplary embodiment of the present invention, a chip handlingapparatus, in particular a die bonder, is presented which comprises achip supply station; a chip mounting station; and one or more chiphandling units configured to pick a chip from the supply station,transport the chip to the mounting station, and place the chip at amounting location; wherein each chip handling unit is configured totemporarily retain the chip in a defined position relative to the chiphandling unit, and wherein the chip handling apparatus further comprisesmeans for inducing sonic vibrations in the chip when retained by one ofthe chip handling units; and means for measuring the vibrations inducedin the chip.

In another exemplary embodiment of the present invention, a chiphandling unit, in particular for use in a semiconductor die bonder, ispresented, which is configured to receive a chip, in particular asemiconductor die, at a takeover location and to hand over said chip toa delivery location, said chip handling unit comprising, means fortemporarily retaining the chip in a defined position relative to thechip handling unit, and means for inducing vibrations, in particularsonic and/or ultrasonic vibrations, in the chip.

In another exemplary embodiment of the present invention, a chiphandling process, in particular for a die bonding process, is presentedwhich comprises the steps of a) receiving a chip (5), in particular asemiconductor die, at a takeover location by means of chip handlingunit; b) temporarily retaining the chip in a defined position relativeto the chip handling unit; c) transporting the chip to a deliverylocation; and d) subsequently releasing the chip, wherein between stepsa) and d), vibrations are induced in the chip; and a vibrations inducedin the chip are measured.

The aforementioned and further objectives, advantages and features ofthe invention will be detailed in the description of preferredembodiments below in combination with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 shows a die bonder in accordance with an exemplary embodiment ofthe present invention.

FIG. 2 shows a schematic of a preferred embodiment of a chip handlingtool for use with the present invention.

FIG. 3 shows a schematic of another preferred embodiment of a chiphandling tool for use with the present invention.

FIG. 4 shows a schematic of yet another preferred embodiment of a chiphandling tool for use with the present invention.

FIG. 5 shows a flow diagram illustrating a chip handling process inaccordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic of an exemplary embodiment of a die bonder inaccordance with the present invention. The die bonder comprises aplurality of chip handling units: A zeroth chip handling unit, alsocalled die ejector 91, ejects a die 5, which may be laminated with a WBLor FOW lamination, from a carrier tape 59 at a wafer table. Wafer tableand die ejector 91 act as chip supply station for die bonder. After thedie 5 has been ejected, it is taken over by a first chip handling unit,also referred to as pick unit 92. The die bonder further comprises asecond chip handling unit also referred to as place unit 94, fortransporting die 5 to and placing it onto a target position on asubstrate 6 at a chip mounting station. The pick unit 92 is capable ofrotating around a first axis, and configured to hand the die 5 over to athird chip handling unit. Said third chip handling unit, also referredto as transfer unit 93, is capable of rotating about a second axis, andmay thus hand the die 5 to the place unit 94. The transfer unit 93comprises a chip handling tool 10, which comprises at least one vacuumorifice for temporarily attaching the die 5 to the chip handling tool10. The transfer unit 93 further comprises a piezo electric actuator,which is configured to induce acoustic and/or ultrasonic vibrations inthe die 5 attached to the chip handling tool 10. When the transfer unit93 is at a delivery location as represented by a solid line in FIG. 1,acoustic and/or ultrasonic vibrations are induced in the die 5 by meansof the piezo electric actuator. A laser displacement meter 7 is providedto measure the vibrations induced in die 5.

The die bonder further comprises a control system not shown in FIG. 1 tocontrol the movements of the various chip handling units etc.

FIG. 2 shows a schematic of a preferred embodiment of a chip handlingtool 10 for use with the present invention. The chip handling tool 10comprises a shank 103 by means of which it may e.g. be mounted to a chiphandling unit, in particular the transfer unit 93. Piezo electricactuator 101 is provided on the shank 103. A tube 102 consisting of aflexible material, e.g. rubber, is mounted on the piezo electric acuator101. The tube has a vacuum supply connection 1021, which allows forconnecting to a vacuum source in order to temporarily retain die 5sucked against a vacuum orifice formed at the top end of tube 102.

FIG. 3 shows a schematic of another preferred embodiment of a chiphandling tool 10′ for use with the present invention. The chip handlingtool 10′ again comprises shank 103 by means of which it may e.g. bemounted to a chip handling unit, in particular the transfer unit 93.Piezo electric actuator 101 is again provided on the shank 103. In thisembodiment, tube 102 is mounted directly on the shank 103. Again, die 5may be sucked against the vacuum orifice formed at the top end of tube102, and thus temporarily retained in position. Dimensions of tube 102and piezo electric actuator 101 are chosen in such a way that a smallair gap, preferably having a length between 25 and 500 μm, preferably 50to 200 μm results between a die sucked against the top end of tube 102and piezo electric actuator 101.

FIG. 4 shows a schematic of yet another preferred embodiment of a chiphandling tool 10″ for use with the present invention. The chip handlingtool 10′ again comprises shank 103 by means of which it may e.g. bemounted to a chip handling unit, in particular the transfer unit 93. Ahollow piezo electric actuator 101′ is provided on the shank 103, sothat a vacuum orifice is formed at an end of the hollow piezo electricactuator 101′ remote from the shank 103. A vacuum supply channel 1031 isformed in the shank to allow for supplying vacuum to an inner side ofhollow piezo electric actuator 101′. Preferably, a layer of softmaterial, e.g. rubber, is provided on the end of the hollow piezoelectric actuator 101′ remote from the shank 103 in order to prevent die5 from coming into contact with a relatively hard material of the piezoelectric actuator 101′.

As already mentioned, laser displacement meter 7 may be used to measurevibrations induced in die 5, e.g. by measuring displacements and/ordeflections of die 5, e.g. as a function of time. The displacementsand/or deflections may be measured at a single location, e.g. in justone corner of the die 5. Preferably, they are measured at two ormultiple locations. This way it is ensured that damages may be detectedirrespective of their location. It also allows to get an indication ofwhere the damage might be located. This in turn is helpful as guidancefor additional die inspection methods that may be applied.

Optical metrology based on interference may also be appliedadvantageously to measure displacements and/or deflections of die 5.

Based on an input to the piezo electric actuator 101 and measureddisplacement and/or deflection, one or more frequency response functionsof the die 5 are calculated. Those contain amplitude and phaseinformation on how the die 5 reacts to a vibration excitation. Ingeneral, the frequency response functions show various resonance peakswhich correspond to natural eigenmodes of the die in that particularexperimental setup.

When the die 5 is damaged, e.g. when cracks, splits, chip offs or otherdefects are present in the die, the frequency response functions showdeviations when compared to reference frequency response functions of anundamaged, but otherwise identical die 5. In a damaged die 5, mechanicalstiffness and mode shapes change, so that e.g. resonance shifts andresonance broadening due to additional damping may be expected. Thus,damaged dies 5 may be identified by comparing measured frequencyresponse functions with the reference frequency function.

This is preferably accomplished by comparing response functions over acontinuous range of frequencies, preferably in a subrange between 10 Hzand 100 kHz. Alternatively, it may be accomplished by just comparing oneor more individual resonances with those from known good dies.Preferably, a sine-wave excitation is used, and the excitation frequencyis sweeped slowly. However, a simultaneous excitation over a range offrequencies could also be used to advantage, in particular to allow forfaster determination of the frequency response function. In particular,one could think of a white noise excitation to excite a wide spectrum offrequencies at once.

Instead of measuring vibrations induced in die 5 based on measurementsof displacement and/or deflection by optical means as described above,an acoustic and/or ultrasonic receiver, e.g. a microphone or anotherpiezo electric element, may preferably be employed. The vibrationfrequency response may thus be obtained directly without needmeasurements of displacement and/or deflection. Preferably, thevibrations induced in die 5 may also be measured by determiningimpedance or an impedance spectrum of the piezo electric actuator.

Based on a discrimination between damaged and undamaged dies 5 asdescribed further above, exception handling controls may then allow foravoiding attaching of broken dies 5, and for removing them from thetransfer unit 93.

FIG. 5 shows a flow diagram in accordance with certain exemplaryembodiments of the present invention. As is understood by those skilledin the art, certain steps included in the flow diagram may be omitted;certain additional steps may be added, and the order of the steps may bealtered from the order illustrated.

In the description so far, die 5 was retained in a defined positionrelative to the transfer unit 93 by means of vacuum suction. Thisimplies that the die 5 is sucked against some kind of support surface inwhich one or more vacuum orifices are formed, or which is defined by oneor more vacuum orifices. Preferably, the die 5 may also be retained inposition by a contactless setup, for example by a combination ofattractive, e.g. electrostatic, forces and repulsive forces, e.g. due aflow of compressed air, to which the die may be exposed in closeproximity to a work surface of an adequately designed chip handlingtool.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

In particular, although the invention was described above with respectto semiconductor dies, it may be used for any kind of chip, includingany essentially a flat slab of material or materials that is picked fromany kind of supply carrier, in particular a tape or belt, and placedonto a target location, in particular on a chip, substrate, tape, beltor a storage means.

What is claimed:
 1. A chip handling apparatus comprising a) a chipsupply station, b) a chip mounting station, c) one or more chip handlingunits configured to pick a chip from the chip supply station, transportthe chip to the chip mounting station, and place the chip at a mountinglocation, wherein each chip handling unit is configured to temporarilyretain the chip in a defined position relative to said chip handlingunit, d) means for inducing vibrations, in particular sonic and/orultrasonic vibrations, in the chip when retained by one of the chiphandling units; and e) means for measuring the vibrations induced in thechip such that at least one measured frequency response function isconfigured for comparison to at least one reference frequency responsefunction to determine whether the chip is damaged.
 2. The chip handlingapparatus according to claim 1, further comprising discrimination meansconfigured to determine whether the chip is damaged based upon ameasurement of the vibrations induced in the chip.
 3. The chip handlingapparatus according to claim 1 wherein the one or more chip handlingunits includes a first chip handling unit for picking the chip from thechip supply station, and a second chip handling unit for placing thechip onto the chip mounting location, and wherein the first chiphandling unit is configured to hand the chip to the second chip handlingunit at a first hand over location.
 4. The chip handling apparatusaccording to claim 1 wherein the one or more chip handling unitsincludes a first chip handling unit for picking the chip from the chipsupply station, a second chip handling unit for placing the chip ontothe mounting location, and a third chip handling unit configured toreceive the chip from the first chip handling unit, and hand it to thesecond chip handling unit.
 5. The chip handling apparatus according toclaim 4, wherein the means for inducing vibrations in the chip areconfigured to induce vibrations when the chip is retained by the secondchip handling unit.
 6. A chip handling unit configured to receive a chipat a takeover location and to hand over said chip to a deliverylocation, said chip handling unit comprising: a) means for temporarilyretaining the chip in a defined position relative to the chip handlingunit, and b) means for inducing vibrations, in particular sonic and/orultrasonic vibrations, in the chip, such that at least one measuredfrequency response function is configured for comparison to at least onereference frequency response function to determine whether the chip isdamaged.
 7. A chip handling process comprising the steps of: a)receiving a chip at a takeover location by means of a chip handlingunit, b) temporarily retaining the chip in a defined position relativeto the chip handling unit, c) transporting the chip to a deliverylocation, d) subsequently releasing the chip, wherein between steps a)and d), vibrations are induced in the chip, and the vibrations inducedin the chip are measured, and wherein at least one measured frequencyresponse function is configured for comparison to at least one referencefrequency response function to determine whether the chip is damaged. 8.The chip handling process according to claim 7, wherein the vibrationsinduced in the chip are sonic and/or ultrasonic vibrations.
 9. The chiphandling process according to claim 7, wherein at the delivery location,the chip is placed onto a substrate or onto another chip.
 10. The chiphandling process according to claim 7, wherein at the delivery location,the chip is handed to a place unit.
 11. A chip handling processcomprising the steps of: a) picking a chip from a supply station, b)transporting the chip to a mounting station by means of at least onechip handling unit, said at least one chip handling unit beingconfigured to temporarily retain the chip in a defined position relativeto the chip handling unit, and c) placing the chip onto a mountinglocation, wherein in step b), vibrations are induced in the chip, and d)the vibrations induced in the chip are measured, and wherein at leastone measured frequency response function is configured for comparison toat least one reference frequency response function to determine whetherthe chip is damaged.
 12. The chip handling process according to claim11, wherein the vibrations induced in the chip are sonic and/orultrasonic vibrations.
 13. The chip handling process according to claim11 further comprising the step of determining whether the chip isdamaged based upon an analysis of the vibrations measured.